RAID 5 expansion loses 2TB

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jrichemont
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RAID 5 expansion loses 2TB

Post by jrichemont »

Hi.

I have tried to expand a 3*4TB RAID 5 to a 5 * 4TB one. I know (now) about the 16TB limit but thought that applied to the final size of the usable array rather than the size of the disk pool it was built from. A RAID 5 array should have 1-(1/n) usable space; for 20TB this gives 16TB; on the limit I thought.

After rebuilding I now have a 5 disk RAID with 14TB usable space. What has happened to the other two TB? Even if the pool could only expand to 16TB and the RAID was build out of there I'd get 12.8TB usable space. To get 14TB needs either an efficiency coefficient of 0.7 (no RAID type I know gives that) or a pool size of 17.5TB which 'loses' 2.5TB. Did it just do its best to expand to 20TB and run out of puff at 17.5?

Aside from that; what should I actually do to get full use of my 5 4TB drives? Can I make 2 RAID arrays from it at 10TB each (8TB usable space each)? I do want redundancy so am unwilling to do stuff like use the first 4 drives for RAID and leave the last one; it simply won't get used for anything as it is less safe and I'll have wasted my money.

All advice appreciated!

Jeremy
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schumaku
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Re: RAID 5 expansion loses 2TB

Post by schumaku »

Ref. 16 vs around 14 TB ... are you aware today's HDD provide storage capacity in decimal, and no longer in binary "old" TB?

Hint (borrowed from Wiki Tebibyte) 1 Tebibyte = 2^40 bytes = 1099511627776 Bytes = 1024 gibibytes

In other words, the (5-1) * 4 TB are effectively something like (5-1) * 3.6 TiB ... what translates to a (binary) 14.4 TiB ... what QNAP (and many OS) are still showing as TB.

The missing storage space is stolen by the HDD vendors - not by QNAP or the NAS ... in fact this started already several years ago, around the first/second generation of TB capacity drives.

Borrowed representatively from WD Red Description:
===
As used for storage capacity, one megabyte (MB) = one million bytes, one gigabyte (GB) = one billion bytes, and one terabyte (TB) = one trillion bytes. Total accessible capacity varies depending on operating environment.
As used for buffer or cache, one megabyte (MB) = 1,048,576 bytes. As used for transfer rate or interface, megabyte per second (MB/s) = one million bytes per second, megabit per second (Mb/s) = one million bits per second, and gigabit per second (Gb/s) = one billion bits per second.
===
jrichemont
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Re: RAID 5 expansion loses 2TB

Post by jrichemont »

Ah, that actually makes sense. I was aware in the "not really uppermost in my mind" kind of way. So what you are saying is my 4TB drive:

would have been: 4,294,967,296 bytes == 4.29TiB (or 4TB in old money)
is actually: 4,000,000,000 bytes == 4TiB (or 3.72TB in old money)

That's assuming we had disks that size back then. OK, so 5 * 3.7 is 18.6 which would give 14.6 with a raid efficiency of 0.8 for raid 5. Hmmm. I suppose 4TB looks better than 3.7TB on the package but still.

I shall blame marketing rather than anything else. I normally do that anyway...

Thanks!
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pwilson
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Re: RAID 5 expansion loses 2TB

Post by pwilson »

jrichemont wrote:Ah, that actually makes sense. I was aware in the "not really uppermost in my mind" kind of way. So what you are saying is my 4TB drive:

would have been: 4,294,967,296 bytes == 4.29TiB (or 4TB in old money)
is actually: 4,000,000,000 bytes == 4TiB (or 3.72TB in old money)

That's assuming we had disks that size back then. OK, so 5 * 3.7 is 18.6 which would give 14.6 with a raid efficiency of 0.8 for raid 5. Hmmm. I suppose 4TB looks better than 3.7TB on the package but still.

I shall blame marketing rather than anything else. I normally do that anyway...

Thanks!
(5 - 1) * 3.6TB = 14.4TB.

Yes, in HDD manufacturer marketing/BS: 1TB = 1,000,000,000 Bytes, despite the fact that this is actually 0.9TB. Plenty of lawsuits over this. Packaging on drives now contains: "1TB = 1,000,000,000 Bytes" in the fine print, to prevent further lawsuits, while continuing to use this same dishonest marketing.

The only good news here is that all manufactures use this same "Math", so at least you can compare drives from different manufactures easily. For now, simply accept that the "formatted capacity" of a 4TB drive is only 3.6TB.

Patrick M. Wilson
Victoria, BC Canada
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